1. Field of the Invention
The present invention relates to semiconductor manufacturing. More specifically, the present invention relates to a method and apparatus for quickly determining the effect of placing an assist feature at a location in the layout.
2. Related Art
Dramatic improvements in semiconductor manufacturing technologies presently make it possible to integrate tens of millions of devices onto a single chip.
Semiconductor manufacturing technologies typically include a number of processes which involve complex physical and chemical interactions. Since it is almost impossible to find exact formulae to predict the behavior of these complex interactions, researchers typically use process models which are fit to empirical data to predict the behavior of these processes. A process model can be used in a number of applications during the design of a semiconductor chip. For example, process models are commonly used for making corrections to layouts to compensate for undesirable effects of semiconductor manufacturing processes.
One of the important steps in semiconductor manufacturing involves placing assist features in a layout. Assist features (AFs) can be printing (e.g., super-resolution assist features) or non-printing (e.g., sub-resolution assist features). In either case, assist features are meant to improve the robustness (e.g., depth-of-focus) of the layout.
Prior art techniques for placing assist features typically use mask rules, which place and cleanup assist features based on combinations of feature width and spacing parameters. Such rule-based approaches can result in missed or sub-optimal placement and/or cleanup of assist features. Further, the complexity of such rules increases rapidly with shrinking features size, thereby requiring more wafer data for calibration and more effort on the part of engineers. Moreover, these rules can be overly restrictive which can prevent designers from being able to achieve the best semiconductor device performance.
Note that we may be able to determine a good location for placing an assist feature if we can quickly simulate the effect of placing an assist feature at a location in a layout. However, prior art techniques perform at least two full-fledged simulations (one without the assist feature and one with the assist feature) to determine the effect of placing the assist feature at a location.
Unfortunately, performing a full-fledged simulation of a layout can require an infeasible amount of computation (cycles and/or time). As a result, prior art simulation techniques usually cannot be used to determine a good location for placing an assist feature in a layout.
Hence, what is needed is a method and apparatus to quickly determine the effect of placing an assist feature at a location in a layout.